Conducting polymers have emerged as important class of electronic materials because of their potential and wide applications in energy storage systems, opto-electronic devices, organic light emitting diodes, sensors for hazardous gases and toxic fumes, corrosion inhibitors for iron and mild steel and EMI shielding in radio frequency range and microwave range. The study of the conducting polymers is a sub field of the larger, older field organic electrical conductors, which had already started in early 1970's, with the discovery of (SN)x. These organic materials possess electronic conductivity comparable to metals and semiconductors. The conducting polymer, polyacetylene, has electronic conductivity of the order of 105 S/cm whereas the conductivity of copper is 106 S/cm. With the idea that electronic conductivity can be varied with doping has revolutionized the area of research. They acquire importance over inorganic semiconductors in their application because of their high strength to weight ratio, toughness, low cost and ease of processing into film. The prospect of plastic metals has inspired much interest in these materials for technological applications such as antistatic coatings and electromagnetic interference shielding and in other areas where light weight, flexibility and high conductivity materials are required.
Among conducting polymers, polyaniline and its analogues have been widely studied due to its ease of protonic acid doping in the emeraldine form and its environmental stability in both doped and undoped forms. Conducting polymer, polyaniline, exists in various forms and each form find technological application. The fully reduced form of polyaniline is leucoemeraldine, which finds applications in Li-polyaniline battery and electrochromic devices. 50% reduced and 50% oxidized form of polyaniline is termed as emeraldine base, which is the insulating form of polyaniline. It finds application in sensors for HCl gas and as corrosion protective coating on iron and mild steel. Doping of emeraldine form leads to the formation of conducting polyaniline, which is used as an electrode material in batteries, sensors, EMI shielding and electrochromic devices. The fully oxidized form of polyaniline is pernigraniline, which find applications in non-linear optics. Conductivity in polyaniline depends on the degree of protonation of the material.
The polymerization of aniline to polyaniline in the presence of organic protonic acids like p-toluene sulphonic acid, dodecylbenzene sulphonic acid and aerosol OT may bring certain changes in the properties of polyaniline because conduction mechanism in polyaniline involves protonation as well as ingress of counter anions to maintain charge neutrality. Protonation and electron transfer in polyaniline leads to the formation of radical cations by an internal redox reaction, which causes the reorganization of electronic structure to give two semiquinone radical cations. In the doping process, ingress of anions occurs to maintain charge neutrality in the resultant doped polyaniline matrix. This implies that nature of anions should influence the properties of the resulting polyaniline. This is the reason why polyaniline doped with inorganic dopants like Cl− and SO42− are thermally less stable than the polyaniline doped with organic dopants like p-toluene sulphonate or dodecylbenzenesulphonate.
Doping of polyaniline with organic protonic acids can be carried out by direct method in which a monomer is polymerized in the presence of organic protonic acid or by indirect method in which emeraldine base is doped with desired organic acid. In both the methods, the dopant is attached with polyaniline. However, the polymer obtained by all these techniques had only electrical conductivity and does not have any magnetic behaviour. In this direction, worldwide researchers are employing different techniques to incorporate magnetic constituents in the polymer backbone so that the resultant polymer possesses both electrical and magnetic properties. However, by introducing ferrite constituents, the resultant polymers obtained possess less conductivity or the magnetization value obtained had less values. By insitu polymerization of monomers in the ferrite medium, we observed that the resultant polymers possesses both electrical conductivity and good magnetization value of the order from 35 to 48.9 emu/gm. The patentibility of the present invention is based on these observations.
Earlier different attempts were made by researchers like making blends with ferrofluids or by blending ferrites in the polymer matrix.
U.S. Pat. No. 5,264,157 relates to the synthesis of an electrically conducting polymer poly pyrrole having magnetic properties. In this, composite material has been prepared by the polymerization of monomer, pyrrole in the presence of a colloidal suspension of charged magnetic particles like ferrofluids.
U.S. Pat. No. 4,604,229 describes a method of preparing ferrofluid composition in a ferrofluid seal apparatus comprising a liquid carrier having a colloidal dispersion of ferromagnetic particles in an amount sufficient to provide magnetic properties to the ferrofluid composition and carbon particles stabilized in the ferrofluid composition by a surface active dispersing agent.
U.S. Pat. No. 6,919,158 provides a method of providing a conductive pattern forming material by which a fine pattern having a high resolution is obtained.
U.S. Pat. No. 6,764,617 claim a formation of conductive ferromagnetic composition comprising sulfonated lignin or a sulfonated polyflavonid or derivatives thereof and ferromagnetic iron oxide particles. The invention provides a conducting ferromagnetic material consisting of iron oxide materials and a conducting polymer comprising lignosulfonic acid doped polyaniline.
U.S. Pat. No. 6,927,666 reports the fabrication of a inductor which comprises a magnetic core that occupies substantially the entire volume enclosed by the conductive coil where the substrate comprises Si, SiC, Ge etc. and magnetic core comprises ferromagnetic material deposited by chemical vapour deposition.
A composite of polyaniline with both conducting and ferromagnetic functions has been reported by Wan and Li, J. Polymer Science (1997) 2129-2136 where the composite of polyaniline has been prepared by varying the concentrations of FeSO4 where a maximum magnetization value, Ms, of 20 emu/g has been observed but the composite show insulating behaviour.
U.S. Pat. No. 5,471,185 (November 1995) reports the electrical circuit protection devices comprising conductive liquid compositions. The invention provides an electrical circuit protection device using a conductive liquid contained in flexible tube contacted and sealed at each end by an annular metal electrode capped by a Flexible membrane. The flexible tube is further sealed inside a solid insulating tube, which contains a ferromagnetic liquid. The conducting liquid has a resistivity of about 1 to 2000 milliohm-cm.
Composite of polyaniline containing iron oxide with nanometer size has been synthesized by a chemical method as reported in Synthetic Metals 78 (1996) 27-31 by M. Wan, W. Zhou and J. Li. For the basic preparation conditions (e.g. pH 14), the resulting PANI-FexOy composite can be attracted by a magnet and its magnetization with the applied magnetic field exhibits a hystersis loop with a Hc=0. However the electrical conductivity of the composite is 10−4 S/cm with a saturation magnetization value of the order of 20 emu/gm.
Magnetic properties of conducting polymer doped with manganese-zinc ferrite nanoparticles has been reported by P. Poddar, J. L. Wilson, H. Srikanth (NRL, USA)-Nanotechnology 15 (2004) S 570-74. The magnetic properties of super paramagnetic particles are influenced by the supporting matrix. A study of the DC magnetic properties of loosely packed manganese-zinc ferrite synthesized using reverse micelle technique were embedded in polypyrrole matrix.
Coexistence of ferromagnetism and metallic conductivity in a molecule based layered compound has been reported by E. Coronado in Nature 408(6811):447-9, 2000. They have synthesized single crystals of a compound composed of layers od BEDT-TTF interleaved with layers of Mn—Cr Oxalate. When positively charged BEDT-TTF has semiconductor properties and their charge balances the negative charge of Mn—Cr-Oxalate. In this Mn is in oxidation state II and Cr in state III and these provide a two dimesional array with ferromagnetic behaviour. The conductivity at room temperature reaches a value of ˜250 S/cm having Ms value of 7.
Improved Conductivity and Electrical Properties of Polyaniline in the presence of rare earth cations and magnetic field has been discussed by L. T. Cai, S. B. Yao & S. M. Zhou in the Synthetic Metals, 88 (1997) 205-208. The influence of rare earth cations on the preparation and properties of polyaniline was investigated. The existence of paramagnetic ions can greatly increase the can greatly increase the effects of magnetic alignment. The conductivity of PANI/Tb3+/Bp=0.7 T was 115.4 S/cm.
U.S. Pat. No. 6,919,063 (July 2005) reports a method of preparing Carbon nanoparticles and transparent conductive polymer composite containing the same. The present invention relates to a novel Carbon nano particle and a novel method of preparing the same and a conductive polymer composite containing the same. Decyltrimethylammonium bromide was added to pyrrole with oxidant FeCl3 added into the reactor. PPy nano particles were then moved into electric furnace and then heated to about 900° C. under N2 environment with the heating rate of 3° C./minute. The electrical conductivity of the present composite is 16×10−4 S/cm and magnetic anisotropic coefficient of the order of 2.4×108 ergs/cm3.
Novel ferromagnetic behaviour of conducting polymers doped with fullerene and TDAE has been reported in Synthetic Metals, 86 (1997) 2333-34. Magnetic behaviour of composites of C60 and some soluble conducting polymers such as poly(3-alkylthiophene) and poly(2,5-dioctyloxyphenylene) has been studied by ESR measurements. Spin-susceptibility depends upon temperature markedly. Dependence of magnetic behaviour on the thermal treatment and molecular structure of host conducting polymer is given.
Method of Obtaining Polymeric Current-Conducting Material has been reported by V. Ye. Gul, N. F. Shchibrya, Foreign Technology Div. Wright-Patterson AFB OH, Report Date: 26 OCT 90, Report number: A921332. Methods of obtaining isotropic polymeric current-conducting materials by introduction to polymer of ferromagnetic filler with after hardening in magnetic field are given. The dispersion of ferromagnetic filler for the bonding agents used are 45.6 d of nickel of carbonyl, 46.7 d 23% solution of the partially saponified copolymer of vinyl acetate with the vinyl chloride of brand A 15-0 in the mixture of organic solvents, 7.7 g DGU (50% solution of diethyleneglycolurethane in cyclohexanone).
Magnetic behaviour of composites containing polyaniline coated manganese zinc ferrite has been reported by N. E. Kazantseva in J. of Magnetism & Magnetic Materials, 269 (2004) 30-37. Polycrystalline Mn—Zn ferrite has been coated with PANI doped with picric acid, which is embedded into a polyurethane matrix. The coated ferrite particles had a conductivity of 0.34 S/cm. The paper provides only Ms value of ferrite, which is 3.5 kGs.
Preparation of magnetic and conductive NiZn ferrite-polyaniline nanocomposites by G. Li, S. Yan, E. Zhou, Y. Chen has been reported in Colloids & Surfaces A: Physicochem (2005). Magnetic & conductive NiZn ferrite nanocomposites with core shell structure have been fabricated by micro emulsion process. Ms value of NiZn ferrite was observed to be 5.84 emu/gm whereas for the composite Ms observed is 0.76 emu/gm. The conductivity of the composite is 0.094 S/cm.